[docbook] What's the difference between complete DTD and simplified one?

[Yves S. Garret]

Anyone know?

Complete:
http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd

Simplified:
http://www.oasis-open.org/docbook/xml/simple/1.0/sdocbook.dtd

I'm new to it and the syntax doesn't really make any sense.